Ion exchange membrane is in wide use as a membrane for solid polymer fuel cell, redox flow cell or zinc-bromine cell or the like, a membrane for dialysis, etc. Solid polymer fuel cell uses an ion exchange membrane as the electrolyte membrane. This fuel cell is an electricity generation system in which a fuel and an oxidizing agent are fed continuously, they are reacted, and the resulting chemical energy is taken out as an electric power; and it is an electricity generation system which is clean and highly efficient. In recent years, the solid polymer fuel cell has increased its importance for uses in automobile, household and portable device because it can be operated at low temperatures and can be produced in a small size.
The solid polymer fuel cell has, in general, a structure in which a gas diffusion electrode having a catalyst loaded thereon is bonded to each side of a membrane composed of a solid polymer functioning as an electrolyte. When electricity is generated using such a solid polymer fuel cell, hydrogen gas or a liquid fuel (e.g. methanol) is fed into a chamber (fuel chamber) in which one of the gas diffusion electrodes is present, and an oxygen-containing gas as an oxidizing agent (e.g. oxygen or air) is fed into a chamber in which the other gas diffusion electrode is present. When, in this state, an external circuit is connected to the two gas diffusion electrodes, the fuel cell works as such and an electric power is supplied to the external circuit.
Of solid polymer fuel cells, direct liquid fuel cell using methanol or the like per se as the fuel, is evaluated highly because it uses a liquid fuel easy to handle and the fuel is inexpensive. Therefore, solid polymer fuel cells are expected as a electric source of relatively small output used especially for portable device.
The fundamental structure of direct liquid fuel cell is shown in the drawing. In the drawing, 1a and 1b are each a partition wall of cell. The cell partition walls 1a and 1b are formed at the both sides of a solid polymer electrolyte membrane 6 used as a membrane so as to sandwich the solid polymer electrolyte membrane 6. 2 is a fuel passage formed in the inner wall of one cell partition wall 1a. 3 is an oxidizer gas passage formed in the inner wall of other cell partition wall 1b. 4 is a diffusion electrode of fuel chamber side. 5 is a gas diffusion electrode of oxidizer chamber side.
In this direct liquid fuel cell, when a liquid fuel such as alcohol or the like is fed into a fuel chamber 7, proton (hydrogen ion) and electron are generated at the fuel chamber side diffusion electrode 4. The proton passes through the inside of solid polymer electrolyte membrane 6 and reaches an oxidizer chamber 8, where the proton reacts with oxygen gas or with the oxygen in air, generating water. The electron generated at the fuel chamber side diffusion electrode 4 passes through an external circuit (not shown) and is sent to the oxidizer chamber side gas diffusion electrode 5. At this time, the external circuit is provided with an electric energy.
In the direct liquid fuel cell having the above-mentioned structure, there is ordinarily used a cation exchange membrane as the solid polymer electrolyte membrane 6. The cation exchange membrane is required to have properties of small electric resistance, high physical strength and low permeability to liquid fuel used. When the cation exchange membrane has a high permeability to liquid fuel, the liquid fuel fed into the fuel chamber migrates to an oxidizer chamber side, resulting in a reduced cell output.
As the cation exchange membrane used as a membrane for fuel cell, there has been known, for example, a membrane in which a cation exchange resin is filled in the pores of a porous membrane made of a polyolefin resin or a fluoroplastic. This cation exchange membrane is produced by a method which comprises filling, in the pores of a porous membrane, a polymerizable composition composed of a polymerizable monomer having a functional group into which a cation exchange group can be introduced and a crosslinkable polymerizable monomer, polymerizing the polymerizable composition to form a resin, and introducing a cation exchange group into the functional group into which a cation exchange group can be introduced (for example, Patent Literatures 1 and 2). The method can produce a membrane for fuel cell at a relatively low cost and the membrane produced is small in electric resistance, low in permeability to hydrogen gas, and low in swelling and deformation caused by solvent.
Patent Literature 1: JP2001-135328 A
Patent Literature 2: JP1999-310649 A